The use of an electric field is relatively new medical treatment for such purposes as the enhancement of chemotherapy (electrochemotherapy), cellular ablation, and intracellular electromanipulation. All three treatments involve pulses of energy, although using different frequencies and pulse durations. Ablation, such as of tumor or liver cells, occurs when cells are exposed to high-voltage electrical field pulses. In the presence of these pulses, the electrochemical potential across the cell membrane is altered and instabilities in the polarized lipid bilayer are induced, which may lead to the development of irreversible pores (or enlargement of existing pores) in the cell membrane. The phenomenon may cause cell death through the loss of cellular contents or entry of surrounding contaminants.
This increase in cell membrane permeability when exposed to an electric field is generally referred to as electroporation, and may be brought about by the application of pulses of direct current (DC) electrical energy applied internally (via, for example, a catheter) or externally. However, the repetition frequency of electric pulses is considered to effect muscle contractions, which can produce a burning sensation or intense pain in patients.
Further, deep lesions are sometimes required to effectively treat some cardiac conditions. For example, atrial fibrillation may be caused by aberrant electrical conductivity pathways through and around scarred myocardial tissue, which cause an electrical “feedback loop” and irregular heartbeat. To destroy these aberrant pathways, the myocardial tissue must be ablated deeply enough to stop the problematic electrical signals from continuing. The creation of deep lesions requires prolonged application of energy and/or high temperatures. However, the surface of the tissue must be maintained at cool enough temperatures to avoid charring or micro embolus formation, which can lead to unintended tissue death or stroke.
Therefore, a system and method are desired that is capable of producing deep lesions without the production of high heat and without causing patient discomfort. The system and method of the present invention involve the application of higher voltage radiofrequency energy to use both heat and electroporation to perform ablation while avoiding undesired tissue damage due to excess heat at the tissue surface.